The chemical activation process can be described essentially as a de-hydration reaction which removes almost quantitatively both the free water and the bound water content of a carbonaceous raw material. The term "bound water", as used herein, means the water of constitution, that is, the hydrogen and oxygen content of the molecules comprising the carbonaceous raw material and the activating agent. The term "free water", as used herein, means the water content present as water molecules in the initial mixture of the raw material and activating agent. The activating agent functions as a dehydrating agent to accomplish the removal of bound water from the carbonaceous raw material. The products of the activation reaction, therefore, are primarily carbon and water. One of the raw materials typically used in this very old and well known process is wood sawdust. However, other carbonaceous materials having a sufficient oxygen content greater than about 25 percent on a moisture and ash free basis can also be advantageously used and references to raw materials as used herein in the context of such materials mean those which are capable of chemical activation. Examples include but are not limited to cellulosic materials, peat and the low rank brown coals and the like. Those having the higher ranges of oxygen content as usually preferred.
As practiced heretofore, the conventional chemical activation process for practical commercial production consists of continuously feeding the impregnated raw material into one end of a direct-fired rotary furnace and discharging activated carbon at the other end. The term "direct-fired", as used herein, is defined as the introduction of combustion gases directly into the furnace to provide the heat energy necessary to accomplish activation. "Combustion gases" as used herein, mean the mixture of the products of combustion, most often natural gas or fuel oil, and secondary air. In this method, heat energy is primarily transferred from the combustion gases by their direct contact with the impregnated raw material to accomplish carbonization and activation. The combustion gases also function to convey, or sweep away, the water and acid produced by the activation reactions. The term "counter-current", as used herein, means that the flow of solid material and the flow of combustion gases occur in opposite parallel directions in the furnace. In this conventional direct-fired activation process, a quantity of fuel must be burned which provides sufficient heat energy to activate the raw material. This heat energy requirement is commonly referred to as the activation heat duty.
In the prior art commercial scale chemical activation processes, the temperature of the combustion gas is lowered from flame temperature by adding secondary or dilution air at a rate sufficient to produce the desired temperature of the furnace inlet gas in a counter-current manner relative to the introduction of the raw material feed. The temperature of the furnace inlet gas is maintained at a level which will raise the temperature of the carbon at the point of discharge from the furnace to a level which will produce the desired residual volatile, or un-carbonized, content of the furnace product.
While the activation level of activated carbon is often expressed on a weight basis, e.g., surface area or absorption capacity per gram of carbon, activated carbon is most often used on a volume basis. That is, a given fixed volume of activated carbon is employed to adsorb liquids or gaseous matter. Therefore it is desirable to be able to control the apparent density of the activated carbon end product so as to maximize its activity on a volume basis.
However, the nature of the conventional, commercially practiced, direct-fired process for making chemically activated carbon fixes certain process parameters in a manner which severely limits the ability to adequately control the treatment of the raw material and hence the ability to optimize the desired properties of the final end product.
Another very significant problem with the commercially practiced prior art processes for chemical activation of carbonaceous materials is that the raw material particles develop a "stickiness" or adhesiveness on their surface. Then the particles form agglomerates and/or adhere to the internal parts of the furnace. This problem is generally most pronounced during the early stages of activation. During this early critical period, the temperature of the raw material typically increases from about 90 degrees C. to about 160 degrees C.
Various methods are used to cope with this problem of adhesiveness, all of which create additional undesirable results. One method replaces flighting with scraper bars in the areas where sticking occurs. The removal of flights decreases the rate of heat transfer since the particles then reside in a bed instead of being lifted and showered through the combustion gases, thereby introducing an uncontrolled variation in the rate of the activation reaction. The method also has the undesirable result of degrading particles which become wedged between the scraper and the furnace wall.
Another method installs chains to either the flights or the furnace wall or both. While the movement of these chains does dislodge adhered particles, it also disintegrates many of these particles. Still another method recycles a portion of the furnace discharge product back into the furnace to absorb the excess accumulation of liquid from the surface of the sticky particles. This method has the disadvantage of reducing the production capacity of the furnace by an amount which is directly proportional to the rate at which carbon is re-cycled, and of producing unknown effects upon the activation properties of the re-cycled carbon.
This problem of excessive adhesiveness is discussed in Canadian Patent No. 842,778. This patent teaches recycling priorly activated carbon into the raw material feed to reduce sticking of the particles to one another or parts of the furnace.
U.S. Pat. No. 2,083,303 utilizes this excessive stickiness to form shaped particles which are then oven hardened and subsequently placed in a conventional rotary kiln to complete carbonization and activation. This process is not considered practically applicable to high volume, cost effective production.
Prior to the present invention, a satisfactory solution to the problems and control limitations associated with prior art chemical activation processes has eluded those skilled in the art. A method of chemically activating carbon which overcomes these problems in a practical and economical manner suitable for producing large commercial volumes of product has not been taught or suggested by the prior art.